1. Field of the Invention
The present invention relates to a method and to a system for accessing of transmission channels of a communication system with at least a central base station, with at least a duplex transmission channel and with a plurality of subscriber stations independent of each other, where each subscriber station can have access to the duplex transmission channel according to a multiple access method.
2. Brief Description of the Background of the Invention Including Prior Art
A system for transmission of messages via radio communication can be recited as an example for such a communication system. A radio transmission system is known from German Patent Application Laid Open DE-OS 2,537,683 having stationary radio stations and mobile subscriber stations. There to each stationary radio station (central base station) is assigned a set of duplex transmission channels. One of the channels, the control channel, is employed for the transmission of control information for system operation and the other channels (the voice channels) are employed for voice communication. In the following, the transmission direction from the subscriber station to the central base station is called forward direction and the reverse transmission direction is called backward direction. The principles of operation of various channel access methods are taught in German Patent Application Laid Open DE-OS 2,537,683.
At the start of each transmission from a subscriber station to the central base station initially an identification of the own mobile station is transmitted in forward direction via the control channel (duplex transmission channel). If two or more subscriber stations emit their identification at the same time onto the control channel, then none of these identifications can be recognized in the central base station except for the special case, where one of the identifications is received with a substantially larger power as compared with all the others.
With increasing number of subscriber stations also the probability increases that more than one of the subscriber stations intend to access the control channel at the same time. Since the control channel is to be used by all subscriber stations jointly, the probability for access attempts at the same time to the control channel and therewith the probability of destruction of the signals are particularly large.
A further increase in the probability of destruction results from the repetition of attempts of accessing based on not recognized identifications. Upon each destruction of an access the transmission channel is not useable at least for the duration of an identification, that means blocked. The portion of time the control channel is blocked due to access destructions increases additionally with the length of the identification, that is with the number of the subscriber stations to be admitted to the complete communications system.
In case of short and frequent transmissions the capacity of the duplex transmission channel can only be utilized to a small part.
Joel, Jr. teaches in U.S. Pat. No. 3,663,762 a mobile communication system where an electronic processor is incorporated. The system comprises a plurality of base stations each located in individual cell areas. Predetermined cell areas are periodically interrogated to detect movement of located mobile stations into new areas.
Wells et. al. in U.S. Pat. No. 3,898,390 teach a mobile radio communication system and a method for increasing the effective use of communication channels in a small zone communication system. Calls between two mobile units may be routed through a central control terminal and base stations. The mobile units may all search for and lock onto an appropriate control signal channel. A call is initiated over a base station control signal channel and the control and signalling link connecting the base station to the control terminal. An available one of the voice channels assigned to the base station through which the call is established is then assigned to the call. In the event that the call was originated by a mobile unit and a call back is requested, the central processing unit may store the call until circuits are clear and then notify the mobile unit of the availability of circuits at a later time (column 7, lines 59 to 63).